This chapter evaluates the suitability of models (in particular coupled atmosphere-ocean general circulation models) for use in climate change projection and in detection and attribution studies. We concentrate on the variables and time-scales that are important for this task. Models are evaluated against observations and differences between models are explored using information from a number of systematic model intercomparisons. Even if a model is assessed as performing credibly when simulating the present climate, this does not necessarily guarantee that the response to a perturbation remains credible. Therefore, we also assess the performance of the models in simulating the climate over the 20th century and for selected palaeoclimates. Incremental improvements in the performance of coupled models have occurred since the IPCC WGI Second Assessment Report (IPCC, 1996) (hereafter SAR) resulting from advances in the modelling of the atmosphere, ocean, sea ice and land surface as well as improvements in the coupling of these components.

Highlights include:

• Coupled models can provide credible simulations of both the present annual mean climate and the climatological seasonal cycle over broad continental scales for most variables of interest for climate change. Clouds and humidity remain sources of significant uncertainty but there have been incremental improvements in simulations of these quantities.
• Confidence in model projections is increased by the improved performance of several models that do not use flux adjustment. These models now maintain stable, multi-century simulations of surface climate that are considered to be of sufficient quality to allow their use for climate change projections.
• There is no systematic difference between flux adjusted and non-flux adjusted models in the simulation of internal climate variability. This supports the use of both types of model in detection and attribution of climate change.
• Confidence in the ability of models to project future climates is increased by the ability of several models to reproduce the warming trend in 20th century surface air temperature when driven by radiative forcing due to increasing greenhouse gases and sulphate aerosols. However, only idealised scenarios of only sulphate aerosols have been used.
• Some modelling studies suggest that inclusion of additional forcings such as solar variability and volcanic aerosols may improve some aspects of the simulated climate variability of the 20th century.
• Confidence in simulating future climates has been enhanced following a systematic evaluation of models under a limited number of past climates.
• The performance of coupled models in simulating the El Niño-Southern Oscillation (ENSO) has improved; however, the region of maximum sea surface temperature variability associated with El Niño events is displaced westward and its strength is generally underestimated. When suitably initialised with an ocean data assimilation system, some coupled models have had a degree of success in predicting El Niño events.
• Other phenomena previously not well simulated in coupled models are now handled reasonably well, including monsoons and the North Atlantic Oscillation.
• Some palaeoclimate modelling studies, and some land-surface experiments (including deforestation, desertification and land cover change), have revealed the importance of vegetation feedbacks at sub-continental scales. Whether or not vegetation changes are important for future climate projections should be investigated.
• Analysis of, and confidence in, extreme events simulated within climate models is emerging, particularly for storm tracks and storm frequency. �Tropical cyclone-like� vortices are being simulated in climate models, although enough uncertainty remains over their interpretation to warrant caution in projections of tropical cyclone changes.

Final Assessment

Coupled models have evolved and improved significantly since the SAR. In general, they provide credible simulations of climate, at least down to sub-continental scales and over temporal scales from seasonal to decadal. The varying sets of strengths and weaknesses that models display lead us to conclude that no single model can be considered �best� and it is important to utilise results from a range of coupled models. We consider coupled models, as a class, to be suitable tools to provide useful projections of future climates.

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